Printing system, method of recording images, and ink cartridge attachable to printing system

ABSTRACT

A printing system of the present invention reduces a total amount of discharge of at least three different color inks that are mixable to express hues in a predetermined range without deteriorating the degree of granularity. The printing system of the invention reads recording ratios of the respective color inks corresponding to input tone data regarding the respective color inks from tables TC, TM, and TY and forms dots according to the recording ratios. Yellow ink Y has a higher dye density than a balancing density that ensures a color balance. This makes the recording ratio of the yellow ink Y lower than the recording ratios of cyan ink C and magenta ink M. The yellow ink Y has high lightness, so that sparely formed dots do not increase the degree of granularity even in an area of low tone data. The enhanced density of the yellow ink Y reduces the total amount of inks discharged from a head to satisfy a required printing density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printing system, which has a head forrecording three or more different inks that are mixable to express huesin a predetermined range and records multi-tone images with the inksdischarged from the head onto a printing object. The present inventionalso pertains to a method of recording images and an ink cartridgeattachable to such a printing system.

2. Description of the Related Art

Color printers with a head for recording a plurality of color inks on aprinting object are widely used as an output device of a computer, whichprints multi-color, multi-tone images processed by the computer. Thefollowing techniques are known as the method of recording inks on aprinting object: a thermal transfer system that melts inks on inkribbons and transfers the molten inks to a sheet of paper; an ink jetsystem that sprays colored ink solutions onto a sheet of paper; and anelectrophotographic system that uses laser or another light to create alatent image on a photoconductive intermediary and transfers colortoners to render the latent image visible. All such available techniquesreproduce colors in a predetermined hue range by mixing a plurality ofcolor inks. Three color inks, cyan, magenta, and yellow (CMY) aregenerally used for full-color printing.

There are several techniques available for printing multi-tone,multi-color images with a plurality of color inks. One availabletechnique, which is adopted in the conventional printers, expresses thetone of a printed image by the density of dots (the frequency ofappearance of dots per unit area) while fixing the size of dots formedon a sheet of paper by a stream of ink droplets. Another availabletechnique adjusts the diameter of dots formed on a sheet of paper, inorder to vary the density of ink per unit area. The advanced fineworking of the head for creating ink particles has improved the densityof dots formable per predetermined length or the variable range of dotdiameters.

The improvement in printers has, however, so far been limited to 300 dpithrough 720 dpi in printing density or resolution and several tensmicrons in particle diameter. This is significantly lower than the levelof expression or resolution of silver photography, which has reachedseveral thousands dpi on the film. Dots are sparsely formed in an areaof low image density, that is, in an area of low density of dots to beprinted. This increases the degree of granularity and makes the dotsundesirably conspicuous. In printers for spraying liquid ink onto apaper, the total amount of ink sprayed per unit area is restricted bythe absorbable volume of ink by the paper (generally referred to as inkduty). The restriction of ink duty is an issue to be cleared in printersusing a plurality of color inks for color printing. The issue of inkduty is actualized especially in case that higher-density ink andlower-density ink are provided for the respective colors and thelower-density ink is used for printing low-tone areas in order todecrease the degree of granularity. Expression of a specific tone withthe lower-density ink results in increasing the total amount of inksprayed per unit area.

SUMMARY OF THE INVENTION

The object of the present invention is thus to regulate the density of aspecific color ink and relieve the restriction of ink duty while keepingthe quality of a printed image in a printing system with a head forspraying three or more different inks that are mixable to express huesin a predetermined range.

At least part of the above and the other related objects are realized bya first printing system having a head for recording at least threedifferent color inks, which are mixable to express hues in apredetermined range, on a printing object, wherein a density ratio of aspecific color ink, which has highest lightness against a fixedrecording ratio among the at least three different color inks, to theother color inks is determined, in order to enable a color balance to bebiased to the specific color ink having the highest lightness in casethat the specific color ink and the other color inks have an identicalrecording ratio per unit area. The first printing system includescorrection means for correcting a recording amount of the specific colorink having the highest lightness to a level that cancels the bias.

The present invention is also directed to a first image recording methodcorresponding to the first printing system. The first method records amulti-tone image by controlling a head that is able to record at leastthree different color inks, which are mixable to express hues in apredetermined range, and regulating a distribution of dots formed by theat least three different color inks based on tone signals regarding anoriginal image to be printed. The first method includes the steps of:

determining a density ratio of a specific color ink, which has highestlightness against a fixed recording ratio among the at least threedifferent color inks, to the other color inks, in order to enable acolor balance to be biased to the specific color ink having the highestlightness in case that the specific color ink and the other color inkshave an identical recording ratio per unit area; and

correcting a recording amount of the specific color ink having thehighest lightness to a level that cancels the bias.

The first printing system (or the corresponding first image recordingmethod) of the present invention has a head for recording at least threedifferent color inks that are mixable to express hues in a predeterminedrange. The printing system forms dots of these color inks at apredetermined ratio, so as to record images of various hues andlightness (densities). In the first printing system, the color balanceis biased to the specific color ink having highest lightness against afixed recording ratio, in case that all the color inks have an identicalrecording ratio per unit area. The correction means then corrects therecording amount of the specific color ink having the highest lightnessto a level that cancels the bias. Such correction ensures a normal colorbalance and reduces the recording amount of the specific color inkhaving the highest lightness and less effect of granularity in a lowdensity area. This structure accordingly reduces the total amount ofinks to be recorded while keeping the quality-of a resulting image.

At least part of the above and the other related objects are alsorealized by a second printing system having a head for recording atleast three different color inks, which are mixable to express hues in apredetermined range, on a printing object, wherein a density ratio of aspecific color ink, which has lowest conspicuousness of granularityagainst a fixed recording ratio among the at least three different colorinks, to the other color inks is determined, in order to enable a colorbalance to be biased to the specific color ink having the lowestconspicuousness of granularity in case that the specific color ink andthe other color inks have an identical recording ratio per unit area.The second printing system includes correction means for correcting arecording amount of the specific color ink having the lowestconspicuousness of granularity to a level that cancels the bias.

The present invention is further directed to a second image recordingmethod corresponding to the second printing system. The second methodrecords a multi-tone image by controlling a head that is able to recordat least three different color inks, which are mixable to express huesin a predetermined range, and regulating a distribution of dots formedby the at least three different color inks based on tone signalsregarding an original image to be printed. The second method includesthe steps of:

determining a density ratio of a specific color ink, which has lowestconspicuousness of granularity against a fixed recording ratio among theat least three different color inks, to the other color inks, in orderto enable a color balance to be biased to the specific color ink havingthe lowest conspicuousness of granularity in case that the specificcolor ink and the other color inks have an identical recording ratio perunit area; and

correcting a recording amount of the specific color ink having thelowest conspicuousness of granularity to a level that cancels the bias.

The second printing system (or the corresponding first image recordingmethod) of the present invention has a head for recording at least threedifferent color inks that are mixable to express hues in a predeterminedrange. The printing system forms dots of these color inks at apredetermined ratio, so as to record images of various hues andlightness (densities). In the second printing system, the color balanceis biased to the specific color ink having lowest conspicuousness ofgranularity against a fixed recording ratio, in case that all the colorinks have an identical recording ratio per unit area. The correctionmeans then corrects the recording amount of the specific color inkhaving the lowest conspicuousness of granularity to a level that cancelsthe bias. Such correction ensures a normal color balance and reduces therecording amount of the specific color ink having the lowestconspicuousness of granularity. This structure accordingly reduces thetotal amount of inks to be recorded while keeping the quality of aresulting image.

Three primary color inks, that is, yellow, magenta, and cyan inks, arepractically adopted as the at least three different color inks in thefirst and the second printing systems. It is also practical to selectyellow as the specific color ink having the highest lightness or thelowest conspicuousness of granularity. In another combination of colorinks, the specific color ink having highest lightness or lowestconspicuousness of granularity should be selected among all the colorinks.

There are a variety of approaches to bias the color balance in thespecified combination of color inks. One possible approach increases thedye density of the specific color ink having the highest lightness orthe lowest conspicuousness of granularity in a range of 1.1 to 4 times abalancing density that ensures a color balance in case that the at leastthree different color inks have an identical recording ratio per unitarea. The increased density of less than 1.1 times can not exert theexpected effects, whereas the increased density of greater than 4 timesworsens the degree of granularity. Simple adjustment of the dye densitycan readily bias the color balance to a desired extent.

Correction of the recording amount of the specific color ink having thehighest lightness or the lowest conspicuousness of granularity may beaccomplished by decreasing the proportion of dots formed by the specificcolor ink or by decreasing the diameter of dots formed by the specificcolor ink.

Another possible approach to bias the color balance in the specifiedcombination of color inks sets the diameter of dots formed by thespecific color ink having the highest lightness or the lowestconspicuousness of granularity to be greater than the diameter of dotsformed by the other color inks. In this case, the correction meanscarries out the required correction by decreasing the proportion of dotsformed by the specific color ink.

A variety of known methods are applicable to record at least threedifferent color inks on a printing object. By way of example, each ofthese color inks is provided as a solution prepared by dissolving ordispersing a dye or a pigment in a solvent, and each solution containingthe dye or the pigment is sprayed from the head onto the printingobject.

In this structure, the correction means corrects the discharge amount ofthe specific color ink. The method of spraying the ink solutionadvantageously forms fine dots at a relatively high speed.

In the printing system using such ink solutions, it is preferable thatat least two types of inks having different densities are provided forthe color inks other than the specific color ink among the at leastthree different color inks. In this structure, the specific color inkhaving the highest lightness or the lowest conspicuousness ofgranularity as well as the at least two types of inks having differentdensities with respect to the other color inks are sprayed from thehead. This structure provides the low-density, light ink for the colorshaving relatively high conspicuousness of granularity, therebypreventing an increase in degree of granularity in a low density area.

In the color combination of yellow, magenta, and cyan, the at least twotypes of inks having different densities are provided for magenta andcyan. The dye density of the lower-density ink with respect to eachcolor is approximately one quarter the dye density of the higher-densityink. This ensures natural effects on a density variation in the placewhere dots of the deep ink are mixed with dots of the light ink.

A variety of techniques are applicable to form dots of the respectivecolor inks. For example, the dither method may be applied to determineformation or non-formation of dots by each color ink. In the dithermethod, a threshold matrix of discrete dither can be used to determinethe on/off state of dots. The threshold matrix of discrete dither welldisperses the dots and thus advantageously improves the degree ofgranularity.

A variety of mechanisms are applicable for dot formation in suchprinting systems. By way of example, the head may have a mechanism fordischarging ink particles under a pressure applied to each ink runningthrough an ink conduit by application of a voltage to a piezoelectricelement arranged in the ink conduit, or a mechanism for discharging inkparticles under a pressure applied to each ink running through an inkconduit by air bubbles that are produced by a supply of electricity to aheating body arranged in the ink conduit.

The present invention is further directed to an ink cartridge attachableto either one of the above printing systems or used in either one of theabove methods, which has a head for recording at least three differentcolor inks, which are mixable to express hues in a predetermined range,on a printing object. The ink cartridge includes the at least threedifferent color inks, which are mixable to express hues in thepredetermined range and stored therein. At least either one of aspecific color ink having highest lightness against a fixed recordingratio and a specific color ink having lowest conspicuousness ofgranularity against the fixed recording ratio among the at least threedifferent color inks has a dye density higher than a balancing densitythat ensures a color balance in case that the at least three differentcolor inks have an identical recording ratio per unit area. A volume ofthe specific color ink stored in the ink cartridge is not greater than avolume of each of the other color inks.

In the printing systems and the corresponding image recording methodsdiscussed above, the specific color ink having highest lightness orlowest conspicuousness of granularity against a fixed recording ratiohas the increased dye density and thereby the decreased volume ofconsumption. The volume of the specific color ink stored in the inkcartridge is accordingly equal to or less than the volumes of the othercolor inks. This enables the respective color inks to be used up atsubstantially the same timings.

In case that at least two types of inks having different densities areprovided for the color inks other than the specific color ink having thehighest lightness or the lowest conspicuousness of granularity, each ofthe at least two types of inks having different densities has a lessvolume of consumption. In this case, the volume of the specific colorink stored in the ink cartridge may be set greater than the volume ofeach of the at least two types of inks having different densities.

These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiment with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates structure of a printer 20 embodying thepresent invention;

FIG. 2 is a block diagram showing structure of a control circuit 40included in the printer 20;

FIG. 3 is a perspective view illustrating structure of a carriage 30included in the printer 20;

FIG. 4 shows an arrangement of color ink heads 61 through 66 in a printhead 28;

FIG. 5 is a perspective view showing a color ink cartridge 70 a;

FIG. 6 shows a mechanism of ink discharge in each of the color ink heads61 through 66;

FIGS. 7A and 7B show a process of discharging ink particles Ip byextension of a piezoelectric element PE;

FIG. 8 is a block diagram showing a series of processes that enable acomputer 90 to print images based on image information;

FIG. 9 shows compositions of color inks used in the embodiment;

FIG. 10 is a graph showing the lightness plotted against the recordingratio of each color ink;

FIG. 11 shows the relationship between the hue and the saturation ofeach ink in the CIELAB space;

FIG. 12 is a flowchart showing a halftone processing routine executed bya halftone module 99;

FIG. 13 is a graph showing the relationship between the input data andthe recording ratio with respect to cyan, magenta, and yellow inks;

FIG. 14 shows weights added to the peripheral pixels, into which anerror is distributed in the process of error distribution;

FIGS. 15A through 15D show formation of dots by yellow ink and magentaink;

FIG. 16 is a flowchart showing a routine for determining formation ofdeep dots;

FIG. 17 is a flowchart showing a routine for determining formation oflight dots;

FIG. 18 is a graph showing the relationship between the tone data andthe recording ratios of light ink and deep ink in the embodiment;

FIG. 19 shows a process of creating dots by light ink and deep ink; and

FIGS. 20A through 20E show another mechanism for discharging inkparticles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One mode of carrying out the present invention is described below as apreferred embodiment. FIG. 1 schematically illustrates structure of aprinter 20 embodying the present invention. Referring to FIG. 1, theprinter 20 has a mechanism for feeding a sheet of paper P by means of asheet feed motor 22, a mechanism for reciprocating a carriage 30 alongthe axis of a platen 26 by means of a carriage motor 24, a mechanism fordriving a print head 28 mounted on the carriage 30 to control dischargeof inks and formation of dots, and a control circuit 40 for transmittingsignals to and from the sheet feed motor 22, the carriage motor 24, theprint head 28, and a control panel 32.

The mechanism for feeding the sheet of paper P has a gear train (notshown) for transmitting rotations of the sheet feed motor 22 to theplaten 26 as well as a sheet feed roller (not shown). The mechanism forreciprocating the carriage 30 includes a sliding shaft 34 arranged inparallel with the axis of the platen 26 for slidably supporting thecarriage 30, a pulley 38, an endless drive belt 36 spanned between thecarriage motor 24 and the pulley 38, and a position sensor 39 fordetecting the position of the origin of the carriage 30.

FIG. 2 is a block diagram illustrating structure of the control circuit40 and peripheral units included in the printer 20. Referring to FIG. 2,the control circuit 40 is constructed as a known arithmetic and logicoperation circuit including a CPU 41, a P-ROM 43 for storing programs, aRAM 44, and a character generator (CG) 45 for storing dot matrices ofcharacters. The control circuit 40 further includes an exclusive I/Fcircuit 50 exclusively working as an interface to an external motor andthe like, a head drive circuit 52 connecting with the exclusive I/Fcircuit 50 for driving the print head 28, and a motor drive circuit 54connecting with the exclusive I/F circuit 50 for driving the sheet feedmotor 22 and the carriage motor 24. The exclusive I/F circuit 50includes a parallel interface circuit and is connected to a computer viaa connector 56 to receive printing signals output from the computer.Output of image signals from the computer will be discussed later.

The following describes a concrete structure of the carriage 30 and theprinciple of discharging ink by the print head 28 mounted on thecarriage 30. FIG. 3 is a perspective view showing structure of thecarriage 30. FIG. 4 is a plan view illustrating nozzles arranged in theprint head 28 set on the lower portion of the carriage 30 for sprayingthe respective color inks. A color ink cartridge 70 a and a black inkcartridge 70 b are attachable to the substantially L-shaped carriage 30shown in FIG. 3. Detailed structure of the color ink cartridge 70 a isshown in FIG. 5. A partition wall 31 separates the black ink cartridge70 b from the color ink cartridge 70 a. Referring to FIG. 4, six colorink heads 61 through 66 for respectively discharging color inks areformed in the print head 28 that is disposed on the lower portion of thecarriage 30. Ink supply pipes 71 through 76 for leading inks from inktanks to the respective color ink heads 61 through 66 are formed uprighton the bottom of the carriage 30 as shown in FIG. 3. When the color inkcartridge 70 a and the black ink cartridge 70 b are attached downward tothe carriage 30, the ink supply pipes 71 through 76 are inserted intoconnection apertures (not shown) formed in the respective cartridges 70a and 70 b.

When the ink cartridge 70 (including the color ink cartridge 70 a andthe black ink cartridge 70 b) is attached to the carriage 30, inks inthe ink cartridge 70 are sucked out by capillarity through the inksupply pipes 71 through 76 and are led to the color ink heads 61 through66 formed in the print head 28 arranged on the lower portion of thecarriage 30 as shown in FIG. 6. In case that the ink cartridge 70 isattached to the carriage 30 for the first time, a pump works to suckinks into the respective color ink heads 61 through 66. In thisembodiment, structures of the pump for suction and a cap for coveringthe print head 28 during the suction are not illustrated nor describedspecifically.

A row of thirty-two nozzles ‘n’ are formed in each of the color inkheads 61 through 66 as shown in FIGS. 4 and 6. A piezoelectric elementPE having excellent response, which is one of electrically distortingelements, is arranged for each row of nozzles ‘n’. FIGS. 7A and 7Billustrate a configuration of the piezoelectric element PE and thenozzles ‘n’. The piezoelectric element PE is disposed at a position thatcomes into contact with an ink conduit 80 for leading ink to the nozzles‘n’. As is known, the piezoelectric element PE has a crystal structurethat is subjected to a mechanical stress due to application of a voltageand thereby carries out extremely high-speed conversion of electricalenergy to mechanical energy. In this embodiment, application of avoltage between electrodes on either ends of the piezoelectric elementPE for a predetermined time period causes the piezoelectric element PEto abruptly extend and deform one side wall of the ink conduit 80 asshown in the drawing of FIG. 7B. The volume of the ink conduit 80 isreduced with an extension of the piezoelectric element PE, and a certainamount of ink corresponding to the volume reduction is sprayed as inkparticles Ip from the ends of the nozzles ‘n’ at a high speed. The inkparticles Ip soak into the sheet of paper P set on the platen 26, so asto print images.

In order to ensure spaces for the piezoelectric elements PE, the sixcolor ink heads 61 through 66 are divided into three pairs on the printhead 28 as shown in FIG. 4. The first pair includes the black ink head61 that is arranged at one end close to the black ink cartridge 70 b andthe cyan ink head 62 that is disposed next to the black ink head 61. Thesecond pair includes the light cyan ink head 63 for cyan ink having thelower density than that of the standard cyan ink supplied to the cyanink head 62 (hereinafter referred to as light cyan ink) and the magentaink head 64. The third pair includes the light magenta ink head 65 formagenta ink having the lower density than that of the standard magentaink supplied to the magenta ink head 64 (hereinafter referred to aslight magenta ink) and the yellow ink head 66. The compositions anddensities of the respective inks will be discussed later.

In the printer 20 of the embodiment having the hardware structurediscussed above, while the sheet feed motor 22 rotates the platen 26 andthe other related rollers to feed the sheet of paper P, the carriagemotor 24 drives and reciprocates the carriage 30, simultaneously withactuation of the piezoelectric elements PE on the respective color inkheads 61 through 66 of the print head 28. The printer 20 accordinglysprays the respective color inks and transfers multi-color images ontothe sheet of paper P. Referring to FIG. 8, the printer 20 printsmulti-color images based on signals output from an image productionapparatus, such as a computer 90, via the connector 56. In thisembodiment, an applications program 95 working in the computer 90processes images and displays the processed images on a CRT display 93via a video driver 91. When the applications program 95 outputs aprinting instruction, a printer driver 96 in the computer 90 receivesimage information from the applications program 95 and the printer 20converts the image information to printable signals. In the example ofFIG. 8, the printer driver 96 includes a rasterizer 97 for convertingthe image information processed by the applications program 95 todot-based color information, a color correction module 98 for causingthe image information that has been converted to the dot-based colorinformation (tone data) to be subjected to color correction according tothe colorimetric characteristics of an image output apparatus, such asthe printer 20, and a halftone module 99 for generating halftone imageinformation, which expresses density of a specified area by theexistence or non-existence of ink in each dot unit, from thecolor-corrected image information. Operations of these modules are knownto the skilled in the art and are thus not specifically described herein principle, though the contents of the halftone module 99 will bediscussed later.

As discussed above, the printer 20 of the embodiment has the print head28 including a plurality of color ink heads, from which the respectivecolor inks are sprayed. As shown in FIG. 9, yellow ink Y and black ink Kdischarged from the corresponding color ink heads 61 and 66 respectivelyinclude 2.7% by weight of Direct yellow 86 and 4.8% by weight of Foodblack 2 as dyes. The print head 28 has the additional heads 63 and 65for light cyan ink and light magenta ink other than the four heads 61,62, 64, and 66 for the standard four color inks K, C, M, and Y. As shownin FIG. 9, light cyan ink and light magenta ink have lower dye densitiesthan those of the standard cyan ink and magenta ink.

Cyan ink of standard density (defined as C1 in FIG. 9) includes 3.6% byweight of Direct blue 199 as a dye, 30% by weight of diethylene glycol,1% by weight of Surfinol 465, and 65.4% by weight of water. Light cyanink (defined as C2 in FIG. 9), on the other hand, includes only 0.9% byweight of Direct blue 99, that is, one quarter the dye density of thecyan ink C1, and 35% by weight of diethylene glycol and 63.1% by weightof water for adjustment of the viscosity. Magenta ink of standarddensity (defined as M1 in FIG. 9) includes 2.8% by weight of Acid red289 as a dye, 20% by weight of diethylene glycol, 1% by weight ofSurfinol 465, and 76.2% by weight of water. Light magenta ink (definedas M2 in FIG. 9), on the other hand, includes only 0.7% by weight ofAcid red 289, that is, one quarter the dye density of the magenta inkM1, and 25% by weight of diethylene glycol and 73.3% by weight of waterfor adjustment of the viscosity. All these inks are regulated to havethe viscosity of approximately 3 [mPa·s]. In this embodiment, therespective color inks are prepared to have substantially the samesurface tension as well as the viscosity of identical level. Thisensures identical control of the piezoelectric elements PE for therespective color ink heads 61 through 66, irrespective of the type ofinks for creating dots.

Among these inks, all the color inks C1, C2, M1, M2, and Y other thanthe black ink K are stored in the color ink cartridge 70 a shown in FIG.5. In this embodiment, the volume of the yellow ink Y is greater thanthe volume of each of the other inks (C1, C2, M1, and M2). Lower-densityink (light ink) and higher-density ink (deep ink) are provided for cyanand magenta, and the volume of the yellow ink Y is smaller than thetotal volume of light ink and deep ink with respect to cyan or magentain the embodiment. The volume of the yellow ink may, however, be equalto the total volume of light ink and deep ink with respect to each coloror equal to the volume of each light ink or deep ink.

FIG. 10 is a graph showing the lightness L* plotted against therecording ratio with respect to the color inks used in the embodiment.The abscissa of FIG. 10 denotes the recording ratio to the recordingresolution of the printer, that is, the proportion of dots printed onthe white sheet of paper P by the ink particles Ip sprayed out of thenozzles ‘n’. The recording ratio=100 represents the state, in which thewhole surface of the sheet of paper P is covered with the ink particlesIp. This embodiment uses yellow ink Y2 having a greater dye density thanthat of standard yellow ink Y1. As clearly seen in the graph of FIG. 10,the yellow ink Y has the highest lightness among the three primarycolors C, M, and Y. Even at the recording ratio of 100%, the lightnessL* of the yellow ink Y is far over 80%. The lightness L* here representsthe lightness in a CIE1976L*a*b* color space (CIELAB space).

In the graph of FIG. 10, the curve of closed circles represents therelationship between the lightness and the recording ratio with respectto the yellow ink Y2, whereas the curve of open circles represents thesame with respect to the yellow ink Y1 of standard density. The dyedensity of the yellow ink Y2 is 1.5 times as high as the dye density ofthe standard yellow ink Y1. The lightness decreases in proportion to the1.5-fold dye density. The lightness of the yellow ink Y1 of standarddensity at the recording ratio of 100% is equal to the lightness of theyellow ink Y2 of higher density at the recording ratio of approximately67%.

The following describes the relationship between the recording ratio,the hue, and the saturation with respect to the color inks. FIG. 11shows the hue and the saturation with varying recording ratios in thecase of printing images on a sheet of paper with three color inks, thatis, yellow, magenta, cyan. The graph of FIG. 11 represents data of a*b*in the CIE1976L*a*b* color space (CIELAB space). In the CIELAB space,the angle from the abscissa denotes the hue and the distance from theorigin (0,0) denotes the saturation.

The graph of FIG. 11 shows variations in hue and saturation with anincrease in recording ratio by 10% each time for the color inks C, M,and Y1 of standard densities (respectively expressed by the opendiamonds, the open squares, and the open circles).

Variations in hue and saturation for the yellow ink Y2 of the embodimentare expressed by the closed circles, wherein the dye density of theyellow ink Y2 is 1.5 times as high as the dye density of the standardyellow ink Y1. The saturation (chromatic purity) of the yellow ink Y2varies at a greater rate than that of the standard yellow ink Y1 with anincrease in recording ratio by 10% each time. The saturation of theyellow ink Y2 at the recording ratio of 66% substantially coincides withthe saturation of the standard yellow ink Y1 at the recording ratio of100%. This characteristic naturally narrows the range of regulating thenumber of dots. In the case of a 10×10 matrix, 0 to 100 dots areregulated for the yellow ink Y1 of standard density, whereas only 0 to66 dots are regulated for the yellow ink Y2 of higher density.

The color of gray is formed by recording the three color inks ofstandard densities at identical recording ratios. The hue of gray isdeviated to the side of yellow when the yellow ink Y2 of the embodimentis recorded at the same recording ratio as those of cyan and magentainks.

In this embodiment, both the lower-density ink and the higher-densityink are applied for the cyan C and magenta M. The lightness of thelower-density ink and the higher density ink satisfies the followingrelationship. The light cyan ink C2 has approximately one quarter thedye density (percent by weight) of the cyan ink C1. The lightness of thelight cyan ink C2 at the recording ratio of 100% is substantially equalto the lightness of the cyan ink C1 at the recording ratio ofapproximately 35%. This relationship is also applicable to the lightnessof the magenta ink M1 and the light magenta ink M2. The proportion ofthe recording ratios of different-density inks giving the identicallightness is specified by the beauty of color mixture in case that thetwo different-density inks are mixed in print. In practice, it isdesirable to adjust the proportion in the range of 20% to 50%. Thisrelationship is substantially equivalent to the adjustment of the dyedensity (percent by weight) of the lower-density ink (the light cyan inkC2 or the light magenta ink M2) to almost one fifth to one third the dyedensity (percent by weight) of the higher-density ink (the cyan ink C1or the magenta ink M1).

In the printer 20 of the embodiment, the halftone module 99 of theprinter driver 96 carries out the processing to form dots of therespective color inks. The printer 20 of the embodiment uses both thehigher-density ink (deep ink) and the lower-density ink (light ink) forcyan and magenta, and formation of dots by the higher-density ink (deepdots) and formation of dots by the lower-density ink (light dots) followdifferent processing routines. The following description first regardsthe process of forming dots by the yellow ink of higher density and thecyan ink and magenta ink of standard densities and additionally theprocess of forming dots by the higher-density ink and the lower densityink. FIG. 12 is a flowchart showing a halftone processing routine forthe C, M, and Y colors carried out by the halftone module 99. In thehalftone process, the same procedures are basically repeated for the C,M, and Y colors.

When the program enters the routine of FIG. 12, at step S100, thehalftone module 99 first receives tone data of the cyan ink C among thetone data of the respective colors C, Y, and M converted by the colorcorrection module 98 shown in FIG. 8. The tone data are expressed as8-bit data and thereby take the values of 0 through 255. The halftonemodule 99 then reads the recording ratio corresponding to the input tonedata from a table TC and determines the on/off state of dots for thecyan ink C at step S110. FIG. 13 shows an example of the table for therespective color inks. A variety of techniques, for example, errordiffusion method and systematic dither method, are applicable todetermine the on/off state of dots for each color ink. The errordiffusion method is adopted in this embodiment. After determining theon/off state of dots based on the density of cyan regarding a targetpixel, the halftone module 99 calculates and diffuses an error at stepS120. In accordance with a concrete procedure, the difference betweenthe true density regarding the target pixel and the density expressed bythe on state or the off state of dots is calculated as an error. Theerror is then distributed to the peripheral pixels in the vicinity ofthe target pixel with predetermined weights. In the printing process byerror diffusion, a density error occurring in a processed pixel isdistributed in advance into peripheral pixels around the processed pixelwith predetermined weights. The processing of step S120 accordinglyreads the corresponding errors and causes the errors to affect thetarget pixel to be printed next. FIG. 14 illustrates a process ofdistributing the error occurring in a processed pixel PP into peripheralpixels with specified weights. The density error is distributed intoseveral pixels after the processed pixel PP in the scanning direction ofthe carriage 30 and in the feeding direction of the sheet of paper Pwith predetermined weights (¼, ⅛, {fraction (1/16)}).

In this embodiment, inks of two different densities, that is, the deepink and the light ink, are actually provided for cyan C and magenta M toform deep dots and light dots. For the better understanding of thecharacteristic of the present invention, that is, the higher density ofthe yellow ink Y, however, it is assumed that dots of cyan and magentaare formed only by the inks C and M of standard densities (correspondingto the higher-density inks C1 and M1) in the discussion based on theflowchart of FIG. 12.

After the above processing for the cyan ink, the same procedure isrepeated for the magenta ink and the yellow ink. The halftone module 99receives tone data of the magenta ink M at step S130, refers to a tableTM in order to determine the on/off state of dots for magenta at stepS140, and calculates and diffuses an error for magenta at step S150. Thehalftone module 99 then receives tone data of the yellow ink Y at stepS160, refers to a table TY in order to determine the on/off state ofdots for yellow at step S170, and calculates and diffuses an error foryellow at step S180. The recording ratio of dots by the yellow ink Y isreduced to approximately two thirds the recording ratios of dots by thecyan ink C and the magenta ink M as shown in FIG. 13.

Since the recording ratio of dots for yellow ink is about two thirds therecording ratios of dots for cyan and magenta inks, yellow dots do notcompletely fill up the printing area even at the maximum tone data.FIGS. 15A through 15D show formation of dots at the maximum tones ofmagenta and yellow (tone data=255). In this example, the discrete dithermethod is adopted to determine the on/off state of dots in a unit matrixof 3×3. FIG. 15A shows formation of dots by the yellow ink Y1 ofstandard density, wherein the recording ratio is equal to 100[%] at themaximum tone data. In the case of the yellow ink Y2 of the embodimenthaving the higher density, on the other hand, only six dots are formedeven at the maximum tone data (=255) as shown in FIG. 15B. FIG. 15Cshows the case of the magenta ink M, wherein 3×3=9 dots are formed. Incase that dots are formed by both the magenta ink M and the yellow inkY2 of higher density, with respect to the three dots, only the magentaink M is sprayed onto the paper as shown in FIG. 15D.

This results from the fact that the yellow ink Y2 has the dye density of1.5 times as high as the dye density of the standard yellow ink Y1. Thenumber of expressible tones by the yellow ink Y2 of higher density istwo thirds that by the standard yellow ink Y1. Since the yellow inkoriginally has high lightness, sparely formed dots in a low-density areaof an original image do not significantly increase the degree ofgranularity. The yellow ink Y2 of high density accordingly reduces thetotal number of dots formed per unit area, that is, the amount of inksprayed onto a unit area, without deteriorating the picture quality dueto the increase in granularity. Each paper has an upper limit (dutylimit) for the total amount of ink dischargeable per unit area. It isaccordingly advantageous that the increase in density of the yellow inkreduces the required amount of ink. By way of example, the compositeblack is realized by 100[%] of cyan ink+100[%] of magenta ink+60[%] ofyellow ink Y2. Compared with the case using the yellow ink Y1 ofstandard density (maximum recording ratio=100[%]) and thereby requiringthe total duty of 300[%], the case using the yellow ink Y2 of higherdensity requires the duty of 260[%]. This gives the margin of 40[%] withrespect to the duty limit of the paper. As another example, dark red isrecorded on a paper having the duty limit of 190[%]. The conventionalmethod using the standard yellow ink Y1 can print dark red by 10[%] ofcyan+100[%] of magenta+100[%] of yellow, resulting in the total duty of210[%]. This method, however, exceeds the duty limit of the paper andshould accordingly use the black ink to satisfy the duty limit as 90[%]of magenta+90[%] of yellow+10[%] of black. In the resulting print,conspicuous black dots having the highest density are sparely formed torealize dark red. This worsens the granularity and deteriorates thepicture quality. The method of the embodiment using the yellow ink Y2 ofhigh density, on the other hand, realizes dark red by 10[%] of cyan,100[%] of magenta, and 67[%] of yellow. The total duty is lower than theduty limit of the paper. This accordingly ensures the favorablegranularity and high picture quality. The method of the embodiment usingthe yellow ink of high density gives the margin with respect to the dutylimit, thereby optimizing the amounts of the respective color inks andensuring the high picture quality.

The method of the embodiment using the yellow ink of high dye densityreduces the required number of dots. The decrease in number of dotsgives the margin in overlap of the respective color inks. There arevarious devices regarding the overlap of inks. The fact thatapproximately one third the area is free from the dots of yellow inkadvantageously increases the degree of freedom in arrangement of dotsformed by overlapping a plurality of colors. The decrease in number ofdots by the yellow ink naturally reduces the mean consumption of yellowink in the process of printing a predetermined area. This results inreducing the required amount of yellow ink stored in the ink cartridge70. The decrease in amount of yellow ink reduces the total weight of theink cartridge 70 or allows an increase in amount of another color ink.This is especially advantageous in case that higher-density ink andlower-density ink are provided for specific colors like this embodiment.

The printer 20 of the embodiment uses higher-density ink andlower-density ink for magenta and cyan. The process of referring to thetable TC and determining the on/off state of dots by the cyan ink C(step S110) and the process of referring to the table TM and determiningthe on/off state of dots by the magenta ink M (step S140) actuallydetermine formation or non-formation of dots with respect to thehigher-density ink and the lower-density ink.

A process of determining formation of deep dots and a process ofdetermining formation of light dots are carried out at both steps S110and S140. These processes fundamentally determine the on/off state ofdeep dots and light dots based on deep level data regarding the deepdots and light level data regarding the light dots. The deep level dataand the light level data corresponding to the input tone data DS areread from the graph of FIG. 18.

For example, in case that the input tone data of cyan represents a solidarea of 50/256, the recording ratio of the deep cyan ink C1 is equal to0%, so that the value of deep level data Dth is equal to zero. In casethat the input tone data represents a solid area of 95/256, therecording ratio of the deep cyan ink C1 is equal to 7%, so that thevalue of deep level data Dth is equal to 18. In case that the input tonedata represents a solid area of 191/256, the recording ratio of the deepcyan ink C1 is equal to 75%, so that the value of deep level data Dth isequal to 191. In the routine of determining the on/off state of lightdots formed by a lighter ink discussed below, the correspondingrecording ratios of the light cyan ink C2 are 36%, 58%, and 0%,respectively.

The following describes the process of determining formation of dots bytaking advantage of the characteristics shown in the graph of FIG. 18.The procedure first determines the on/off state of deep dots based onthe input tone data DS. This follows a routine of determining formationof deep dots shown in the flowchart of FIG. 16. When the program entersthe routine of FIG. 16, the halftone module 99 first refers to the tableshown in FIG. 18 and reads deep level data Dth corresponding to theinput tone data DS at step S222. FIG. 18 is a table showing therecording ratios of light ink and deep ink plotted against the tone dataof the original image. The tone data DS take the values of 0 to 255 foreach color (8 bit-data for each color), and the magnitude of the tonedata is accordingly expressed, for example, as 16/256 in the followingdescription.

The deep level data Dth is then compared with a predetermined thresholdvalue Dref1 at step S224, in order to determine the on/off state of deepdots. Although the error diffusion method is adopted in this embodiment,a dither method may be applied to determine the on/off state of dots. Inthe latter case, the systematic dither method using a threshold matrixof discrete dither is preferably adopted to set the threshold value. Thethreshold matrix of discrete dither may be, for example, a wide-rangematrix of 64×64 in size (blue noise matrix). In this case, the thresholdvalue Dref1 used as the criterion for determining the on/off state ofdeep dots takes different values for the respective target pixels. Thediscrete dither ensures the high spatial frequency of dots determined bythe threshold matrix and makes dots sufficiently scattered in anyspecific area. A concrete example of the discrete dither is a Beyer'sthreshold matrix. Application of the discrete dither causes deep dots tobe sufficiently scattered and realizes a non-biased distribution of deepdots and light dots, thereby improving the picture quality.

In case that the deep level data Dth is greater than the threshold valueDref1 at step S224, the program determines the on state of deep dots inthe target pixel and calculates a resulting value RV at step S226. Theresulting value RV corresponds to the density of the target pixel(evaluation value of deep dots). In the on state of deep dots, that is,when it is determined that dots of higher-density ink are to be formedin the target pixel, the value corresponding to the density of the pixel(for example, the value 255) is set as the resulting value RV. Theresulting value RV may be a fixed value or set as a function of deeplevel data Dth.

In case that the deep level data Dth is not greater than the thresholdvalue Dref1 at step S224, on the contrary, the program determines theoff state of deep dots, that is, no formation of dots by thehigher-density ink in the target pixel, and sets the value ‘0’ to theresulting value RV at step S228. Since the white background of the sheetof paper P remains in the place where no dots of the higher-density inkare formed, the resulting value RV is set equal to zero.

After determining the on/off state of deep dots, the program enters aroutine of determining formation of light dots shown in the flowchart ofFIG. 17, in order to determine the on/off state of light dots. At stepS240, the halftone module 99 calculates light dot data Dx used fordetermining the on/off state of light dots and obtains corrected data DCby adding a diffusion error ΔDu diffused from the processed pixels tothe light dot data Dx. The light dot data Dx is obtained by thefollowing equation:

Dx=Dth·Z/255+Dtn·z/255

wherein Dtn denotes light level data read from the table of FIG. 18,based on the tone data DS, Z represents an evaluation value in the caseof the on state of deep dots, and z denotes an evaluation value in thecase of the on state of light dots. The light dot data Dx is a sum ofthe deep level data and the light level data respectively multiplied byweighting coefficients, which depend upon the respective evaluationvalues. One of the main characteristics of the present invention is thatthe on/off state of light dots is determined by the light dot data Dx,which is based on both the deep level data and the light level data.Since the evaluation value Z of deep dots is equal to 255, the aboveequation is rewritten as:

Dx=Dth+Dtn·z/255

which is actually used to calculate the light dot data Dx at step S240.The evaluation value z of light dots is significantly smaller than theevaluation value Z of deep dots and is set equal to 160 in thisembodiment.

At subsequent step S242, it is determined whether or not deep dots arein the on state, that is, whether or not deep dots are formed (forexample, by the cyan ink C1). In case of no formation of deep dots, theprogram carries out the processing of and after step S244, in order todetermine the on/off state of the lower-density dots or light dots (forexample, by the light cyan ink C2).

In this embodiment, the error diffusion method is adopted to determinethe on/off state of light dots, that is, formation or non-formation oflight dots, for example, by the light cyan ink C2. At step S244, thetone data DC corrected according to the concept of error diffusion iscompared with a predetermined threshold value Dref2 for light dots. Thethreshold value Dref2 represents a reference value for determiningwhether or not dots should be formed by the low-density, light ink in atarget pixel. Here the threshold value Dref2 is a variable varying withthe corrected data DC.

In case that the corrected data DC is greater than the threshold valueDref2 at step S244, the program determines the on state of light dotsand calculates a resulting value RV (evaluation value of light dots) atstep S246. The resulting value RV here takes the value ‘122’ as areference value and is corrected by the corrected data DC. In case thatthe corrected data DC is not greater than the threshold value Dref2 atstep S244, on the contrary, the program determines the off state oflight dots and sets the value ‘0’to the resulting value RV at step S248.

FIGS. 19a through 19 h show examples of the printing state of light dotsand deep dots formed, for example, by the cyan ink C1 and the light cyanink C2. In the range of low tone data (in the range of tone data=0/256to 63/256 in this embodiment), dots are formed only by the light cyanink C2 as shown in FIGS. 19a and 19 b. The proportion of light dotsexisting in a predetermined area increases with an increase in tonedata, while formation of deep dots starts and gradually increases asshown in FIGS. 19c through 19 e. In the range of higher tone data, nolight dots but only deep dots are formed as shown in FIGS. 19f and 19 g.When the tone data reaches the maximum, the recording ratio of deep dotsis equal to 100% as shown in FIG. 19h.

In the printer 20 of the embodiment discussed above, the ink cartridge70 mounted on the carriage 30 includes the yellow ink of higher dyedensity and the higher-density ink and the lower-density ink for bothcyan and magenta. In the area where the input image has low tones, thelight cyan ink and the light magenta ink having the lower dye densitiesare used for recording. This improves the granularity in the low tonearea, thereby ensuring the extremely high printing quality. The densityof yellow ink can be heightened to the extent that does not worsen thegranularity with respect to dots of yellow ink. The limit is four timesthe standard dye density. This structure significantly reduces the meanvolume of yellow ink discharged per unit area.

Although the density of yellow ink is heightened in the aboveembodiment, the principle of the present invention is not restricted tothe yellow ink. The same effects can be exerted by increasing thedensity of any ink having the highest lightness or the lowestconspicuousness of granularity among the combination of color inks usedfor printing. The embodiment lowers the proportion of dots formed by theyellow ink, in order to correct the biased color balance due to theenhanced dye density of the yellow ink. The bias may alternatively becorrected by decreasing the diameter of dots formed by the yellow ink.The size of dots formed on the sheet of paper P is adjusted byregulating the diameter of each nozzle for spraying ink or the intensityof the voltage pulse (voltage and duration) applied to the piezoelectricelement PE. By way of example, the nozzle 66 for the yellow ink Y2 maybe formed as the nozzle for smaller diametral dots, whereas the nozzles62 and 63 for the cyan ink C and the nozzles 64 and 65 for the magentaink M are formed as the nozzles for greater diametral dots.

In the above embodiment, the programs for controlling formation of therespective color dots are stored in the printer driver 96 included inthe computer 90. These programs may, however, be stored in the printer20. For example, in case that the computer 90 sends image informationwritten in a language, such as PostScript, the printer 20 has a halftonemodule and other required elements. In the embodiment, the softwarerealizing these functions is stored in a hard disk of the computer 90and incorporated into the operating system in the form of the printerdriver at the time of activation of the computer 90. In accordance withanother possible application, the software may be stored in portablestorage media (carriable storage media), such as floppy disks andCD-ROMs, and transferred from the portable storage media to the mainmemory of the computer system or an external storage device. Thesoftware may further be transferred from the computer 90 to the printer20. Still another possible application utilizes an apparatus forsupplying the software via a communication line. In this structure, thecontents of the halftone module may be transferred to either thecomputer 90 or the printer 20 via the communication line.

In the above embodiment, a predetermined voltage is applied to thepiezoelectric elements PE for a predetermined time period, in order todischarge both the low-density ink and the high-density ink. Anothermethod may, however, be applicable to discharge inks. The availableink-discharge techniques can be classified into two types; that is, themethod of separating ink particles from a continuous jet stream of inkand the on-demand method applied in the above embodiment. The formertype includes a charge modulation method that separates droplets of inkfrom a jet stream of ink by means of charge modulation and a micro-dotmethod that utilizes fine satellite particles produced in the process ofseparating large-diametral particles from a jet stream of ink. Thesemethods are applicable to the printing system of the present inventionthat utilizes inks of different densities.

The on-demand type, on the other hand, produces ink particles requiredin the unit of dots. An available method of the on-demand type, otherthan the method utilizing the piezoelectric elements applied in theabove embodiment, uses a heating body ET disposed in the vicinity ofnozzles NZ of ink, produces bubbles BU by heating ink, and makes inkparticles IQ discharged by the pressure of the bubbles BU as shown inFIGS. 20A through 20E. Such on-demand type methods are applicable to theprinting system of the present invention that utilizes inks of differentdensities or plural types of dots having different diameters. The ideaof increasing the density of the ink having the highest lightness or thelowest conspicuousness of granularity is also applicable to thermaltransfer color printers and electrophotographic color printers, such aslaser printers.

The present invention is not restricted to the above embodiment, butthere may be many modifications, changes, and alterations withoutdeparting from the scope or spirit of the main characteristics of thepresent invention.

By way of example, several means disposed in the casing of the printerin the above embodiment may be included in an output apparatus foroutputting images to be printed. The correction means may be realized bya discrete circuit or by the software in an arithmetic and logicoperation circuit including a CPU. In the latter case, for example, theapparatus for outputting images to be printed, such as a computer,carries out the processes relating to the formation of dots, whereasonly the mechanism for controlling discharge of inks from the heads andactually forming dots on a paper is disposed in the casing of theprinter.

There are several variations in structure of the ink cartridge. Forexample, when the printing system is used for color printing with aplurality of color inks, three or more different color inks may bestored in a separate vessel from the vessel of black ink. This structureprevents the timing of replacement of the color ink cartridge from beingaffected by consumption of black ink that is frequently used forprinting characters It should be clearly understood that the aboveembodiment is only illustrative and not restrictive in any sense. Thescope and spirit of the present invention are limited only by the termsof the appended claims.

What is claimed is:
 1. A printing system for recording at least three different color inks, which are mixed to express hues in a predetermined range, on a printing object, wherein a density ratio of a specific color ink, which has highest lightness at a fixed recording ratio of a number of dots per unit area among said at least three different color inks, to the other color inks is determined, in order to enable a color balance to be biased to said specific color ink having the highest lightness in case that said specific color ink and the other color inks have an identical recording ratio of a number of dots per unit area, said printing system comprising: a printing head which prints with said color inks on said printing object; a correction controller configured to correct a recording amount of said specific color ink having the highest lightness to a level that cancels said bias.
 2. A printing system in accordance with claim 1, wherein said at least three different color inks comprise yellow, magenta, and cyan, and said specific color ink having the highest lightness is yellow.
 3. A printing system in accordance with claim 1, wherein said specific color ink having the highest lightness has an increased dye density in a range of 1.1 to 4 times a balancing density that ensures the color balance in case that said at least three different color inks have an identical recording ratio per unit area.
 4. A printing system in accordance with claim 1, wherein said correction controller corrects the recording amount of said specific color ink having the highest lightness by decreasing a proportion of dots formed by said specific color ink.
 5. A printing system in accordance with claim 1, wherein said correction controller corrects the recording amount of said specific color ink having the highest lightness by decreasing a diameter of dots formed by said specific color ink.
 6. A printing system in accordance with claim 1, wherein each of said at least three different color inks recorded on said printing object is provided as a solution prepared by either one of two ways: dissolving either one of a dye and a pigment in a solvent and dispersing either one of said dye and said pigment in said solvent, each said solution containing either one of said dye and said pigment being sprayed from said head onto said printing object, said correction controller correcting a discharge amount of said specific color ink.
 7. A printing system in accordance with claim 6, wherein at least two types of inks having different densities are provided for the color inks other than said specific color ink among said at least three different color inks provided as said solutions, said at least two types of inks having different densities with respect to the other color inks as well as said specific color ink having the highest lightness being sprayed from said head.
 8. A printing system in accordance with claim 7, wherein said at least two types of inks having different densities are provided for magenta and cyan, and a dye density of a lower-density ink with respect to each color is approximately one quarter a dye density of a higher-density ink.
 9. A printing system in accordance with claim 6, wherein said head comprises a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by application of a voltage to a piezoelectric element disposed in said ink conduit.
 10. A printing system in accordance with claim 6, wherein said head comprises a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by air bubbles that are produced by a supply of electricity to a heating body disposed in said ink conduit.
 11. A printing system in accordance with claim 1, said printing system further comprising: means for determining either one of formation and non-formation of dots with respect to each said color ink by a dither method.
 12. A printing system in accordance with claim 11, wherein the dither method utilizes a threshold matrix of discrete dither.
 13. A printing system for recording at least three different color inks, which are mixed to express hues in a predetermined range, on a printing object, wherein a density ratio of a specific color ink, which has lowest conspicuousness of granularity at a fixed recording ratio of a number of dots per unit area among said at least three different color inks, to the other color inks is determined, in order to enable a color balance to be biased to said specific color ink having the lowest conspicuousness of granularity in case that said specific color ink and the other color inks have an identical recording ratio of a number of dots per unit area, said printing system comprising: a printing head which prints with said color inks on said printing object; a correction controller configured to correct a recording amount of said specific color ink having the lowest conspicuousness of granularity to a level that cancels said bias.
 14. A printing system in accordance with claim 13, wherein said at least three different color inks comprise yellow, magenta, and cyan, and said specific color ink having the lowest conspicuousness of granularity is yellow.
 15. A printing system in accordance with claim 13, wherein said specific color ink having the lowest conspicuousness of granularity has an increased dye density in a range of 1.1 to 4 times a balancing density that ensures the color balance in case that said at least three different color inks have an identical recording ratio per unit area.
 16. A printing system in accordance with claim 13, wherein said correction controller corrects the recording amount of said specific color ink having the lowest conspicuousness of granularity by decreasing a proportion of dots formed by said specific color ink.
 17. A printing system in accordance with claim 13, wherein said correction controller corrects the recording amount of said specific color ink having the lowest conspicuousness of granularity by decreasing a diameter of dots formed by said specific color ink.
 18. A printing system in accordance with claim 13, wherein each of said at least three different color inks recorded on said printing object is provided as a solution prepared by either one of two ways: dissolving either one of a dye and a pigment in a solvent and dispersing either one of said dye and said pigment in said solvent, each said solution containing either one of said dye and said pigment being sprayed from said head onto said printing object, said correction controller correcting a discharge amount of said specific color ink.
 19. A printing system in accordance with claim 18, wherein at least two types of inks having different densities are provided for the color inks other than said specific color ink among said at least three different color inks provided as said solutions, said at least two types of inks having different densities with respect to the other color inks as well as said specific color ink having the lowest conspicuousness of granularity being sprayed from said head.
 20. A printing system in accordance with claim 19, wherein said at least two types of inks having different densities are provided for magenta and cyan, and a dye density of a lower-density ink with respect to each color is approximately one quarter a dye density of a higher-density ink.
 21. A printing system in accordance with claim 18, wherein said head comprises a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by application of a voltage to a piezoelectric element disposed in said ink conduit.
 22. A printing system in accordance with claim 18, wherein said head comprises a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by air bubbles that are produced by a supply of electricity to a heating body disposed in said ink conduit.
 23. A printing system in accordance with claim 13, said printing system further comprising: means for determining either one of formation and non-formation of dots with respect to each said color ink by a dither method.
 24. A printing system in accordance with claim 23, wherein the dither method utilizes a threshold matrix of discrete dither.
 25. A printing system for printing at least three different color inks, wherein the at least three different color inks are mixed to express hues in a predetermined range on a printing object, the printing system comprising: a first ink reservoir containing a specific color that has highest lightness at a same recording ratio of a number of dots per unit area among the at least three different color inks; at least two second ink reservoirs containing the at least two color inks other than the specific color ink, respectively, wherein a concentration ratio of the specific color ink to the at least two color inks is determined such that a color balance is biased to the specific color ink when the specific color ink and the at least two color inks have an identical recording ratio of a number of dots per unit area in order to reduce a total amount of the color inks on the object; a correction controller configured to reduce a recording amount of the specific color ink to a level that cancels the bias; and a printing head discharging the at least three color inks on the printing object.
 26. A printing system for printing at least three different color inks, wherein the at least three different color inks are mixed to express hues in a predetermined range on a printing object, the printing system comprising: a first ink reservoir containing a specific color that has the lowest conspicuousness of granularity at a same recording ratio of a number of dots per unit area among the at least three different color inks; at least two second ink reservoirs containing the at least two color inks other than the specific color ink, respectively, wherein a concentration ratio of the specific color ink to the at least two color inks is determined such that a color balance is biased to the specific color ink when the specific color ink and the at least two color inks have an identical recording ratio of a number of dots per unit area in order to reduce a total amount of the color inks on the object; a correction controller configured to reduce a recording amount of the specific color ink to a level that cancels the bias; and a printing head discharging the at least three color inks on the printing object.
 27. An ink cartridge attached to a printer for recording different color inks, which are mixed to express hues in a predetermined range, said ink cartridge comprising: a yellow ink reservoir containing a yellow ink; a cyan ink reservoir containing a cyan ink; a magenta ink reservoir containing a magenta ink; wherein a density of ratio of the yellow ink to the cyan and magenta inks is determined, to enable a color balance to be biased to the yellow ink in case the yellow, cyan, and magenta inks have an identical recording ratio of a number of dots per unit area.
 28. An ink cartridge in accordance with claim 27, wherein an amount of said yellow ink contained is equivalent to or less than an amount of said other inks contained.
 29. An ink cartridge in accordance with claim 28, wherein said yellow ink has an increased dye density in a range of 1.1 to 4 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 30. An ink cartridge in accordance with claim 29, wherein said yellow ink has an increased dye density of about 1.5 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 31. An ink cartridge in accordance with claim 27, wherein said yellow ink has an increased dye density in a range of 1.1 to 4 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 32. An ink cartridge in accordance with claim 31, wherein said yellow ink has an increased dye density of about 1.5 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 33. An ink cartridge attached to a printer for recording different color inks, which are mixed to express hues in a predetermined range, said ink cartridge comprising: a yellow ink reservoir containing a yellow ink; a cyan ink reservoir containing a ink; a magenta ink reservoir containing a magenta ink; a light cyan ink reservoir containing a light cyan ink, wherein a density of the light cyan ink is lower than a density of the cyan ink; a light magenta ink reservoir containing a light magenta ink, wherein a density of the light magenta ink is lower than a density of the magenta ink; wherein a density of ratio of the yellow ink to the cyan and magenta inks is determined, to enable a color balance to be biased to the yellow ink in case the yellow, cyan, and magenta inks have an identical recording ratio of a number of dots per unit area.
 34. An ink cartridge in accordance with claim 33, wherein an amount of said yellow ink contained is more than an amount of said cyan, magenta, light cyan, and light magenta inks contained.
 35. An ink cartridge in accordance with claim 34, wherein said yellow ink has an increased dye density in a range of 1.1 to 4 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 36. An ink cartridge in accordance with claim 35, wherein said yellow ink has an increased dye density of about 1.5 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 37. An ink cartridge in accordance with claim 33, wherein said yellow ink has an increased dye density in a range of 1.1 to 4 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area.
 38. An ink cartridge in accordance with claim 37, wherein said yellow ink has an increased dye density of about 1.5 times a balancing density that ensures the color balance in case that said yellow, cyan, and magenta inks have an identical recording ratio per unit area. 